In a number of scientific applications, it is important to obtain accurate readings of very low and/or rapidly fluctuating air flow rates while retaining a large dynamic range. Unfortunately, conventional air flow sensors are neither sensitive to small flow rates nor do they have fast response time. Further, when used in meteorological applications, harsh environments exist which have hampered the disign of a satisfactory instrument.
One apparatus for overcoming the past deficiencies mentioned above is described in U.S. Pat. No. 4,506,553. In that apparatus, the difference between a dynamic wind pressure at an orifice facing into the wind and a reference pressure is measured. These dynamic and static pressures are then converted to acoustical waves to be detected by a microphone which, with acoustical and electronic processing techniques, provides a very sensitive output. The conversion is implemented by mechanically alternating between two ports respectively connected to the dynamic pressure orifice and the reference pressure orifice. The resulting signal is an acoustical wave. The signal is then passed into a microphone and then processed electronically for a readout indication. The entire acoustical system is designed to obtain the requisite high sensitivity and time response. Sensitivity may be increased by using phase-sensitive amplification of the electrical signal from the microphone. The previously mentioned mechanical switching between two ports is accomplished by utilizing a rotor with passages formed therethrough. A standard photo-optic rotor pickup detects the rotational rate and phase of the rotor which corresponds to the rotational rate and phase of electrical signals due to the acoustical signals picked up by the microphone. Utilizing the photo-detector output as a second input to the phase sensitive amplifier, amplification of only the AC component of the signal, which has a fixed phase relationship to that of the rotor, is processed. A readout is connected to the output of the amplifier for indication of flow rates with significant sensitivity and fast response time.
Thus in the prior art, the alternator was in some form of rotor through which the pressure was transmitted to the microphone system and this alternator carried a significant volume of air. This type of system caused a vacuum pumping action to take place and this led to small but unwanted acoustical signals. The drives for the alternator also caused bearing noise and shake which would be picked up by the microphone even when it was isolated. Additionally, when the response bandwidth was increased to obtain fast response (as in turbulence measurements), any of these incoherent noises decreased the sensitivity of the instrument.